http://tinyurl.com/366d93p
Just thought this was interesting. Found this article today while eating lunch. First, I personally was not surprised by the fact that Atheists and Agnostics scored much higher than religious folks as most of the atheists and agnostics I know (myself included) tend to be more open minded when studying the religions of the world; so having a greater knowledge makes sense.
Surprising to me was how poorly devout Catholics performed as they tend to be some of the most vocal and out going in "sharing" their version and interpretation of religion (the only group I know that is more vocal would be Evangelicals, but they are also crazy so I typically don't take them seriously).
Towards the end of the article there are a few possible reasons, mostly that once people are set in their particular religion they tend to stop studying religion so much, but I'm not entirely sure that I buy this explanation as that doesn't explain why atheists no more about Catholicism than Catholics and more about Protestantism than Protestants (though it would explain why Atheists know more about Catholicism than Protestants and vice-versa).